The experienced error is related to one of the NuGet packages for our latest R2 2018 release - ASP.NET Core, when accessed through the Telerik NuGet feed.
The problematic NuGet package has been temporarily removed from the feed and the feed should be operational. Until we resolve this completely, you can download and use the NuGet package for ASP.NET Core from your Telerik account.
All other NuGet packages for R2 2018 should be working as expected.